Different sites require certain versions of scripts to run. Right now, site maintainers do this by downloading a copy of the library to serve from their site. This is fine if the site is simple and self-contained, but if you add several portlets to your site (a couple ad-rotators, some captchas, social site scripts), pretty soon you run the risk of accidentally including several copies of the same library in your page, all from different domains.
This sops up an enormous amount of bandwidth. Rich pages routinely download 1-2 Mb per page load... this is only going to get worse.
Because the browser would be taking a more active role in managing scripts for separate domains in the same store, some work along the lines of Chrome needs to be done to make sure that script execution across multiple site clients remains secure and robust. Chrome is already leading the way on this, so I think it's doable.
A robust, integrated package management system for browsers would reduce bandwidth costs and at least double or triple the loading speed of AJAX web applications. That's something everyone would like.